Sin is any action or thought that is contrary to the holiness of God and everyone is guilty of sinning. Romans 3:23 - "For ALL have sinned, come short of the glory of God."
Because God is holy (separate from sin), He must punish sin. Sin is what separates us from God. God cannot let us into heaven with our sin. The payment for that sin is an eternity in hell. Romans 6:23 - "For the wages of sin is death..."
John 3:16 says that God loved us so much that He sent His only Son, Jesus, to pay the penalty for our sins so that we might have eternal life. 1 John 1:7 - ...the blood of Jesus Christ His Son cleanseth us from all sin."
We cannot work our way to Heaven. Ephesians 2:8-9 tell us that our salvation from sin comes only by faith, not by works of righteousness. Acts 4:12 tells us that there is NO OTHER WAY to get to Heaven except through Jesus Christ. It is only when we see ourselves as sinners, recognize that Christ died for our sins, and then trust in Him as our Savior that we will be saved from our sins. Acts 16:31 - "...believe on the Lord Jesus Christ, and thou shalt be saved..."